The 連用形(ren'yōkei, “continuative or stem form”) of obsolete verb 境う(sakau, “to divide, to form a boundary between things”).[2][1]

Some sources[2] further derive the verb as Old Japanese element 境(saka, “division, border, boundary”) + auxiliary verb ふ(fu) indicating repetition or ongoing state. However, given the semantics, this could also be analyzed as the repetitive or ongoing aspect of Old Japanese-derived verb saku with an underlying meaning of “to split apart, to separate, to put distance between”, spelled variously as 割く, 裂く, 離く, or 放く, and cognate with 咲く(saku, “to bloom”, from the idea of the blooms splitting open), 栄える(sakaeru, “to prosper”, extended from the bloom sense), 盛ん(sakan, “prospering, fluorishing”).